This short document provides some details on the porpoise movement model developed as an alternative to assess harbour porpoise exposure to wind farms in the North Sea.
Code and data to reproduce the analyses summarized in this document can be found in this repository.
Simplified the approach presented in Joy et
al. (2022).
We simulate the movement of n_agents from March 1 to Nov
30, following the whale densities.
In general terms, there are four steps of the approach:
Step 1: for each agent, choose an initial position
in day t.
Step 2: given position in day t, propose candidate positions for day t+1 by using random draws of step length and turning angle
Step 3: calculate weight for each member of the forecast ensemble. Calculate the probability of each candidate position for each agent
Step 4: sample from the set of candidate positions, with the probability in step 3.
Density maps are from Gilles et al. (2016). Anita Gilles provided the shape files to Magda Chudzinska. They present seasonal density maps of harbour porpoise in the Noth Sea. Note that they cover spring (March-May), summer (Jun-Aug), fall (Sep-Nov). Therefore our movement model is restricted to March-November.
To make sure we have porpoises distributed across the study area in the first step, I divided the space into 10 Voronoi blocks of equal area, calculated the expected proportional abundance in each of the blocks and allocated the agents to each of the blocks proportionally to the expected abundance. Within the blocks, I randomly chose the location of the agents, with the probability of the location proportional to the density of each pixel.